Magnetic cores and methods of constructing same

ABSTRACT

Magnetic cores of the stacked type, and methods of constructing same, having stepped-lap joints between adjoining leg and yoke portions of the core. The length dimensions of the leg and yoke laminations are changed in opposite directions from layer to layer of the magnetic core, while maintaining the midpoints of the laminations in each leg and yoke position in alignment. This arrangement offsets the ends of the laminations from one another, at each end of each leg and yoke portion, in a predetermined stepped pattern, with the stepped patterns of adjoining leg and yoke portions being complementary to provide the desired steppedlap joints.

United States Patent Millsop et al.

[54] MAGNETIC CORES AND METHODS OF CONSTRUCTING SAME [72] Inventors:William Q. Millsop, Muncie, lnd.; Belvin B. Ellis, Pulaski; Charles E.Burkhardt, Sharon, both of Pa.

[73] Assignee: Westinghouse Electric Corporation, Pittsburgh,Pa.

[22] Filed: May 14,1971

[21] App1.No.: 143,429

[is] 3,670,279 51 June 13, 1972 3,540,120 l1/1970 De Laurentis et al..336/21 7 X Primary Examiner-Thomas J. Kozma Attorney-A. T. Stratton,F. E. Browder and Donald R. Lackey ABSTRACT terns of adjoining leg andyoke portions being complementary to provide the desired stepped-lapjoints.

17 Claims, 13 Drawing Figures 52 [15.01 ..336/2l7, 29/609 51 lnt.Cl...H0lf27/04,H01f4l/02 5s FieldofSearch ..336/212,2l6,217;29/609,

29/607 56 References Cited UNlTED STATES PATENTS 2,929,038 3/1960Sonesson ..336/2l7X PATENTEDJUH 1 a 1972 SHEET 10F 3 1 MAGNETIC CORESAND METHODS OF CONSTRUCTING SAME BACKGROUND OF THE INVENTION 1. Field ofthe Invention The invention relates in general to magnetic cores, andmore specifically to magnetic cores of the stacked type for electricalinductive apparatus, such as transformers and electrical reactors,having stepped-lap joints between adjoining leg and yoke portions of thecore.

2. Description of the Prior Art US. Pat. Nos. 3,153,215, 3,210,709,3,477,053 and 3,540,120, which are assigned to the same assignee as thepresent application, all disclose magnetic core structures of thestacked type having stepped-lap joints between adjoining leg and yokeportions of the core. A stepped-lap joint is defined as the type ofjoint wherein the joints between adjoining leg and yoke laminations areincrementally offset from similarly located joints, from layer to layerthrough the stacked core in a predetermined stepped pattern, with thejoints of the pattern progressing for at least three steps in a givendirection before the pattern is repeated or the stepping directionchanged. The stepped-lap joint construction was,

found to improve the performance of a magnetic core, cornpared with thebutt-lap type joint, such as disclosed in US.

Pat. No. 2,300,964, which patent is also assigned to the same assigneeas the present application. The stepped-lap construction reduces thecore losses (T.W.), the exciting-volt amperes (A.W.) and noise level,compared with magnetic cores constructed with joints according to theteachings of the prior art, and it facilitates the construction of themagnetic core as the laminations may be handled and assembled in groupsof laminations.

In general, the prior art stepped-lap joint arrangements, as disclosedin the hereinbefore mentioned patents, obtain the desired steppedrelationship between diagonally cut ends of the laminations by providinglaminations for each leg or yoke portion which have the samelongitudinal dimension between the diagonally cut ends. The steppedrelationship is achieved by incrementally offsetting the midpoints ofthe laminations of any stacked group of laminations, providing a steppedrelationship between the ends of the laminations, on each end of thegroup, but the steps appear on opposite sides of the group. Thus, inassembling a group of incrementally offset laminations, with two othergroups of laminations, such as assembling a group of yoke laminationswith two spaced groups of leg laminations, it is necessary to tuck oneend of the group of yoke laminations under the ends of one of the groupsof leg laminations. This blind assembly of stepped ends increases theassembly time of the magnetic core, it may necessitate flexing orbending of the group of laminations in order to tuck the ends of theyoke laminations under the ends of the leg laminations, which may createstresses in the laminations and adversely affect the magnetic quality ofthe laminations, and a frictional drag is created between the yokelaminations being tucked under the ends of the leg laminations, whichmay cause an undesirable shifting of previously placed laminations.

Thus, it would be desirable to provide a new and improved magnetic corestructure, and methods of constructing same, having stepped-lap jointsbetween adjoining leg and yoke portions of the magnetic core, which doesnot require blind assembly of any of the stepped-lap joint structures.

SUMMARY OF THE INVENTION tern, if the leg laminations are increasing inlength from layer to layer, the corresponding yoke laminations willdecrease in length, and vice versa. This arrangement places the steppedends of any group of laminations on the same side of the group. Thus,the leg portions of the magnetic core may be constructed in groups oflaminations in which the stepped ends on both ends of the groups faceupwardly, outwardly, or otherwise towards the assembler. The steppedends of the groups of adjoining yoke laminations then may be placeddirectly over the stepped ends of the groups of leg laminations, withoutany tucking under of laminations required.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understoodand further advantages and uses thereof more readily apparent, whenconsidered in view of the following detailed description of exemplaryembodiments, taken with the accompanying drawings, in which:

FIG. 1 is a view, in section, which illustrates the prior artarrangement of constructing stepped-lap joints between adjoining leg andyoke portions of the magnetic core;

FIG. 2 is a view, in section, which illustrates the construction ofstepped-lap joints between adjoining leg and yoke portions of themagnetic core, according to the teachings of the invention;

FIG. 3 is a plan view which illustrates the assembly of a magnetic coreaccording to the teachings of the invention;

FIGS. 3A, 3B and 3C illustrate three different joint arrangements whichmay be used with the magnetic core shown in FIG. 3;

FIGS. 4A, 4B and 4C are cross sectional views of the stepped-lap jointarrangement shown in FIG. 3C, taken in the direction of arrows IV-IV,illustrating a single group of adjoining leg and yoke laminations, twosuperposed groups oriented in a similar manner, and two superposedgroups oppositely oriented, respectively;

FIG. 5 is a plan view illustrating a method of orienting the midpointsof a group of laminations having different length dimensions betweentheir diagonally cut ends;

FIG. 6 is a plan view of a magnetic core constructed according toanother embodiment of the invention, and illustrating different ways oforienting the laminations to provide the stepped joint construction;

FIG. 7 is a plan view of a magnetic core constructed according to stillanother embodiment of the invention; and

FIG. 7A is a fragmentary view which illustrates an alternate embodimentof the magnetic core structure shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings,and FIG. 1 in particular, there is shown a cross sectional view of atypical prior art arrangement 9 for constructing stepped-lap joints.Groups 10 and 12 of laminations, such as laminations 14 in group 10,represent two spaced leg portions of a magnetic core, and group 16 oflaminations represents a yoke portion which is to interconnect the endsof groups 10 and 12. Group 16 includes laminations 18, 20, 22 and 24which are like dimensioned between their diagonally cut or mitered ends,with the stepped relationship between the diagonally cut ends beingestablished by incrementally shifting the centerlines of midpoints 26,28, 30 and 32 of laminations 18, 20, 22 and 24, respectively. Thisincremental shifting of lamination midpoints steps the diagonally cutends of the laminations, at both ends of the group 16, providing astepped relationship 34 on one side of group 16, and a steppedrelationship 36 on the other side of the group. The stepped end 34 ofgroup 16 may be easily as sembled with stepped end 38 of group 10, as itmay simply be placed in position, but the assembly of stepped end 36 ofgroup 16 with stepped end 40 of group 12 is more difficult, as steppedend 36 will have to be tucked under the outwardly extending ends ofstepped end 40 in order to butt the diagonally cut ends of thelaminations of group 16 with those of group 12.

If the magnetic core shown in FIG. 1 is of the shell-form type, the viewin FIG. 1 will be an elevation, as the laminations are normally stackedwith their major planes horizontal, necessitating bending or flexing ofgroup 16 in order to assemble the groups, or sliding the laminations infrom the side. The larger overlaps between joints on the largeshell-form magnetic cores, which are required to increase the shortcircuit mechanical strength of the magnetic core, such as one-fourth tothree-eighths inch, makes the stacking problem even more difficult. Ifthe magnetic core shown in FIG. 1 is of the coreform type, FIG. 1 wouldbe a top or plan view, and group 16 would have to be flexed, or thelaminations of group 16 would have to be placed in the same planes asthe laminations of groups and 12, and advanced into the position fromthe top or bottom of the core, depending upon whether group 16 is fromthe top or bottom yoke portions of the core. If it is from the bottomyoke portion, the holding fixture will preclude sliding the group oflaminations in from the bottom, and if it is the top yoke, it would beawkward to handle the laminations and slide them in from the top,especially on the larger core-form type magnetic cores.

FIG. 2 is a cross sectional view of a new and improved arrangement 42for constructing magnetic cores, which arrangement eliminates thedisadvantages of the prior art arrangement shown in F IG. 1. Groups 44and 46 of laminations, such as laminations 48 in group 44, represent twospaced leg portions of a magnetic core, and group 50 of laminations 52,54,

, 56 and 58 represent a yoke portion which is to interconnect the endsof groups 44 and 46.

Laminations 52, 54, 56 and 58 of group 50, instead of having likedimensions between their diagonally cut ends, and incrementally ofisetmidpoints, such as taught by the prior art, have different or varyinglengths between their diagonally cut ends from layer to layer, and themidpoints of the laminations are aligned in each portion of the magneticcore, such as indicated by center line 60. Further, the laminations ofeach group are arranged according to length, with the longest laminationbeing on one side of the group and the shortest on the other side. Forexample, group 50 starts with its longest lamination 52 on one side ofthe group and progresses through v the laminations according to length,with the shortest lamination 58 being on the opposite side of the group,providing progressively stepped arrangements 62 and 64 at opposite endsof group 50, with both arrangements being located on the same side ofthe group. Groups 44 and 46 are constructed similar to group 50, butoriented with their stepped ends facing the stepped ends of group 50,with group 44 having a stepped end 66 which is complementary to steppedend 62 of group 50, and group 46 having a stepped end 68 which iscomplementary to stepped end 64 of group 50. Thus, group 50 may beeasily assembled with groups 44 and 46 by merely placing group 50 in thedesired position over the ends of groups 44 and 46. The assembly shownin FIG. 2 may be constructed without tucking any laminations under otherlaminations. Flexing or bending of laminations is eliminated. Theassembly is not blind", and there is no frictional drag on previouslyplaced laminations which would force these laminations out of position.

FIG. 3 illustrates a magnetic core 70, which is in the process of beingconstructed according to a first embodiment of the invention. If themagnetic core 70 shown in FIG. 3 is of the shell-form type, the viewshown would be a plan view, and if it is of the core-form type, it wouldbe an elevational view. It is to be understood that the magnetic coresshown in the drawings, and the methods disclosed for constructing them,may be either shell-form, or core-form.

Magnetic core 70 includes first and second portions 72 and 74, which maybe leg or yoke portions, depending upon the specific construction of themagnetic core and its intended use, and first and second portions 76 and78, which may also be leg or yoke portions. Each portion of magneticcore 70 includes a stack of superposed, metallic laminations, with eachstack of laminations being made up of one or more groups of laminations.For purposes of example, it will be assumed that each group oflaminations includes four laminations, but in practice more laminationswill usually be used in each group. The number of laminations in a groupdepends upon the number of steps desired in the stepped-lap jointbetween adjoining leg and yoke portions of the core.

More specifically, portion 76 has first, second, third and fourthlaminations 80, 82, 84 and 86, portion 78 has similarly shaped anddimensioned laminations, since it is similar to portion 76 in function.Like reference numerals are used to indicate like dimensionedlaminations in portions 76 and 78, with a prime mark added to thereference numerals of portion 78. Portion 72 of magnetic core 70 hasfirst, second, third and fourth laminations 88, 90, 92 and 94, andportion 74, which is similar in function to portion 72, has laminationswith the same reference numerals as those in portion 72, with theaddition of a prime mark.

The step of providing the laminations for magnetic core 70 may beperformed by cutting the laminations from a strip of magnetic, metallicmaterial, such as grain oriented silicon steel, using a shear which cutsthe strip diagonally at predetermined locations to provide laminationshave a substantially trapezoidal configuration, with the diagonally cutends forming the non-parallel sides of the trapezoid, and the edges ofthe strip forming the parallel sides of the trapezoid. Instead ofcutting all of the leg laminations to the same configuration anddimensions, and instead of cutting all of the yoke laminations to thesame configuration and dimensions, the lengths of the punchings orlaminations for both the leg and yoke portions are varied. The shearused to cut the strip diagonally may be programmed to vary the lengthsof the laminations as required by the specific stepped-lap patternconstruction desired.

The different length laminations for each portion of magnetic core 70 isillustrated relative to portion 76, with lamination 80 having a lengthdimension 96 between its diagonally cut ends. The dimension betweendiagonal ends is defined as that dimension between the ends located onthe shortest side of the two parallel sides of the trapezoidalconfiguration, as the dimension of the longest side may be altered afterthe lamination is cut, such as by incrementally cutting the laminationtips in order to more easily obtain the stepped relationship between agroup of laminations. The length dimension 96 of lamination 80 is theshortest of the group, with lamination 82 having a dimension 98 which islonger than dimension 96. The difference in the lengths of laminations80 and 82 is determined by the amount of overlap desired in thestepped-lap joint structure to be formed between adjoining yoke and legportions of the core. In like manner, lamination 84 has a dimension 100which is longer than dimension 98, and lamina- I tion 86 has a dimension102 which is longer than dimension 100. The remaining portions 72, 74and 78 have the lengths of their respective laminations varied similarto portion 76.

After providing the laminations for the yoke and leg portions of themagnetic core 70, the laminations for each leg and yoke portion arestacked in superposed groups, with the laminations in each group beingsequenced according to length. In other words, the shortest laminationis on one side of each group and the longest lamination is on the otherside, with the laminations between these two laminations increasing inlength from the shortest to the longest lamination. Further, instead ofoffsetting the midpoints or centerlines of the laminations, whichmidpoint or centerline divides the lamination length into two equalportions, the midpoints or centerlines of the laminations in each groupare all aligned with one another, which therefore spaces the diagonallycut ends of the laminations from one another at each end of the group,with the progressive steps of the resulting stepped structure appear.-ing on the same side of each group. Thus, portion 76 has steppedarrangements 104 and 106 at its ends, portion 78 has steppedarrangements 108 and 110 at its ends, portion 72 has steppedarrangements 112 and 114 at its ends, and portion 74 has steppedarrangements 116 andl 18 at its ends.

The stepped arrangement at each end of each group may be easily achievedby incrementally clippinG at least one end of each lamination of eachgroup, on the same end of the group, and aligning the clipped ends, suchas by placing them against a plane surface while maintaining theparallel edges of the group of laminations in alignment. The groupsshown in FIG. 3 have both ends of their respective laminationsincrementally clipped, but for establishing the desired steppedrelationship between the diagonal ends of the laminations, it isnecessary to clip only one end. The clips on the other end will preventthe points on this end of the group from protruding past the major sidesof the magnetic core when certain types of comer joints are utilized.-If the protruding ends will not cause interference in the intendedassembly, they may be left unclipped. For example, in core-formconstruction, protruding ends may interfere with end frames, while inshell-form construction protruding ends may interfere with form-fit typetanks.

In the assembly of portions 72, 74, 76 and 78, two of the paralleldisposed portions, such as portions 76 and 78, are placed such thattheir stepped arrangements 104, 106, 108 and 110 face the assembler. Ina shell-form core the magnetic core is built up around the windings, andit would not be important as to whether the steps on portions 76 and 78face the assembler, or the steps on portions 72 and 74. With a coreformcore, the leg portions should be constructed such that 7 their stepsface the assembler, as it will facilitate the assembly of the top andbottom yokes.

After the groups which form portions 76 and 78 are positioned in spaced,parallel relation, the groups which form portions 72 and 74 may beplaced into position, without flexing or bending the groups, with thesteps of portions 72 and 74 facing away from the assembler to providecomplementary steps between the leg and yoke portions of the core. Thestepped arrangements of the leg and yoke portions fit together toprovide diagonal stepped-lap joints, with the diagonally cut edges ofeach layer of laminations abutting together. It will be noted from FIG.3 that starting with the uppermost layer of laminations, and progressingdownwardly through the layers, that the lengths of the leg and yokelaminations vary in opposite directions, i.e., the laminations inportions 72 and 74 decrease in length between their diagonally cut ends,while the laminations of portions 76 and 78 increase in length betweentheir diagonally cut ends.

When the yoke and leg portions of magnetic core 70 are assembled, theadjoining leg and yoke portions may be placed relative to one another asshown in the fragmentary view of magnetic core 70 in FIG. 3A, whichplaces the geometrical outer corner of the magnetic core 70 at the point120, which is a comer of the portion 74. However, it will be noted thatthis arrangement of portions 74 and 76 generates a substantial voidvolume at the associated inner comer 122. Since the overlap dimension isnecessarily shown large, relative to the other dimensions of thelaminations in the Figures, in order to adequately illustrate thesteppedlap construction, the void volume is magnified out of proportionin the drawings, compared with actual construction, but it doesillustrate that the flux will be forced outwardly adjacent to the innercorner 122, increasing the length of the mean flux path, causing fluxcrowding and higher losses. As shown in FIG. 3B, portions 74 and 76 maybe moved relative to one another along the diagonal joints between them,in a manner as disclosed in the hereinbefore mentioned US. Pat. No.3,477,053, to provide an inner corner 122 which has less void volume,and an outer comer 120' which has more void volume, and the geometricallocation of the outer comer 120 has been moved between portions 74 and76. The transfer of a portion of the void volume to the outer corner isbeneficial, as the normal tendency of the magnetic flux is to hug theinner comer, with the outer corners of the core having a relatively lowflux density. FIG. 3C, illustrates a still greater relative movementbetween portions 74 and 76 along their diagonal joints, to balance thevoid volume between the inner and outer comers 122" and 120respectively, and to also balance the void volume at the FIG. 4A is across sectional view of the stepped-lap joint shown in FIG. 3C, taken inthe direction of arrows lV-IV. The stepped-lap joint shown in FIG. 4A isconstructed of a single group of leg laminations and a single group ofyoke laminations, with this first assembly of groups being referenced124. The next assembly of groups of leg and yoke laminations, which isreferenced 126, may be constructed and oriented in the same manner asthe first assembly 124, which will repeat the stepped-lap pattern of thefirst assembly 124. This arrangement is shown in FIG. 43. Or, the secondassembly 126 may be turned over, relative to the position of the firstassembly 124, such as shown at 126' in FIG. 4C, and this arrangementreverses the stepping pattern of the first assembly.

In certain stepped-lap joint arrangements, the number of laminations ina basic stepped-lap pattern, and the dimensions of the overlap, may notmake it practical to incrementally clip all of the ends of thelaminations at one end of a group of laminations, and align theseclipped ends against a common plane. FIG. 5 is a plan view of a simplefixture 130 which may be used to quickly align the laminations 132, 134,136 and 138 of a group of laminations, taking advantage of thesymmetrical stacking of the varying length laminations, wherein theircenterlines or midpoints are aligned. Fixture 130 includes first andsecond wall portions 140 and 142, the ends of which are joined at comer144, with the angle between the two wall portions being 90. The variouslaminations, after they are cut on a programmed shear, may beautomatically directed to the proper fixtures for the leg and yokeportions. The laminations will be automatically centered in this rightangle fixture, aligning the midpoints or centerlines of the laminationsdisposed therein along the centerline 146.

Figure 6 illustrates a magnetic core with a stepped-lap jointconstruction having 7 layers of laminations in the basic pattern, whichillustrates that a portion of the laminations of a basic group may beincrementally clipped, and a portion of the laminations unclipped.Magnetic core 150 includes first and second spaced parallel portions 152and 154, which may be leg or yoke portions, depending upon the type ofapparatus the magnetic core is to be used with, and first and secondportions 156 and 158 which join the ends of portions 152 and 154 toprovide a substantially rectangular structure defining a closed magneticcircuit which has four outer corners and associated inner comers. Eachportion includes seven laminations, such as laminations 158, 160, 162,164, 166, 168 and 170 of portion 152. These laminations vary in length,and are stacked according to length with their midpoints aligned.Laminations 166, 168 and 170 are incrementally clipped, and laminations158, 160, 162 and 164 are unclipped. Since the laminations of the basicgroups cannot all be assembled and their stepped relationship developedby aligning incrementally clipped ends, the incremental clips are shownmerely to prevent their ends from extending beyond the edges of themagnetic core, and these ends may be left unclipped if they will notcause interference in the assembly of the magnetic core.

FIG. 6 illustrates different ways of developing the desired steppedarrangement in each group. For example, portion 156 may have a roomtemperature curable adhesive 172 applied to each lamination as it isbeing directed into fixture 130 shown in FIG. 5. Alignment of theparallel edges of the laminations as they are removed from the fixtureand allowing the adhesive to cure, will enable the laminations to behandled as a coherent group. The adhesive may also be applied to theedges of the laminations after the group is properly assembled, asdisclosed in the hereinbefore mentioned US. Pat. No. 3,210,709.

Portions 152 and 154 have recesses or notches 174 and 176, respectively,formed in an edge of each lamination of their grespective groups, which,when aligned, will provide the required stepped relationship between theends of the larninations, and portion 158 hasan opening or recess 178disposed completely through each lamination intermediate its paralleledges, which when aligned provides the required stepped relationship ateach end of the group. Notches and circular openings in the laminationsare disclosed in the hereinbefore mentioned US. Pat. No. 3,153,215. Itwill be noted that stepped-lap joints between the leg and yoke portionsof magnetic core 150 appear on both sides of the geometrical outercorners of the magnetic core, and that the relative locations of theadjoining leg and yoke portions, along their diagonal joints, have beenselected to balance the void volume at the inner corners between the legand yoke portions of the core.

FIG. 7 illustrates a magnetic core 180, similar to magnetic core 150shown in FIG. 6, except modified to illustrate an embodiment of theinvention wherein all of the laminations are incrementally clipped, withone-half of the laminations in each group of laminations having theirincrementally clipped ends aligned, and the other half of thelarninations in each group having their incrementally clipped endsaligned, with the ends of the two groups being spaced from one another.More specifically, magnetic core 180 includes first and second parallelportions 182 and 184 which may be leg or yoke portions, and first andsecond portions 186 and 188 which join the ends of portions 182 and 184,to provide a substantially rectangular structure having four outercorners and associated inner corners, which defines a closed magneticloop for use in shell-form inductive apparatus, or core-form apparatus,as desired. Each basic group of laminations in each portion of themagnetic core 180 have seven laminations, with portion 182 havinglarninations 190, 192, 194, 196, 198, 200 and 202. The laminations ofportion 182 have different lengths between their diagonally cut endportions, and they are stacked according to length with their midpointsaligned, providing a stepped relationship on each end of the group, andon the same side of the group. The longer laminations 198, 200 and 202have their ends incrementally clipped and aligned in a common plane 204,and the shorter laminations 190, 192, 194 and 196 have their endsincrementally clipped and aligned along a plane 206, with the lines orplanes 204 and 206 being offset or spaced from one another. Thisarrangement enables each group to be initially handled as two separategroups, aligning each portion of the group with its clipped ends againsta plane surface to develop the desired stepped relationship, and theother half of the group having its clipped ends aligned to establish thedesired stepped relationship between its laminations. The two halves ofthe group may then be placed together, with their midpoints aligned, toprovide a complete group, which is then placed inv position in themagnetic core. While both ends of the longer laminations are shownclipped in the embodiment of the invention shown in FIG. 7, it is onlynecessary to clip one end, unless the other ends will interfere with theassembly of the magnetic core. FIG. 7A is a fragmentary view of themagnetic core 180 shown in FIG. 7, and given the reference numeral 180in FIG. 7A. FIG. 7A illustrates the adjoining portions 182 and 188, withportion 182 modified and given the reference numeral 182', to illustratethat the longer laminations need not be clipped on both ends.

In summary, there has been disclosed new and improved magnetic corestructures of the stacked type, and methods of constructing same. Thenew and improved magnetic cores have stepped-lap joints betweenadjoining leg and yoke portions, which are generated by changing thelength dimensions of the leg and yoke laminations in oppositedirections, from layer to layer across the magnetic core. This enablesthe magnetic cores to be assembled easier and faster, as the steppedends of the two leg portions being joined by two yoke portions, face theassembler. Thus, it is not necessary to bend or flex the groups of yokelarninations being placed into portion, as there is no tucking under ofends required or blind placement of ends, such as encountered in priorart structures. This new and improved arrangement also eliminatesfrictional drag between the laminations being placed and alreadypositioned laminations, eliminating the possibility of moving alreadypositioned laminations.

We claim as our invention:

1. A method of constructing a magnetic core having first and second legportions, first and second yoke portions and stepped-lap joints betweenthe leg and yoke portions, comprising the steps of:

providing a plurality of laminations having a predetermined number ofdifferent length dimensions, with each of the laminations havingparallel edges and mitered edges to define a substantially trapezoidalconfiguration, providing a plurality of groups of laminations byselecting predetermined different lengths of laminations for each group,and stacking the laminations of each group according to length, with thelongest and shortest larninations on opposite sides of each group, andwith their parallel edges and midpoints aligned to space the ends of thelaminations from one another, at each end of the group, in aprogressively stepped arrangement,

orienting two of the groups of laminations to provide the first andsecond leg portions of the magnetic core, with the shortest larninationsof the two groups facing in a predetermined direction,

orienting two of the groups of laminations to provide the first andsecond yoke portions of the magnetic core, with the shortest laminationsof these two groups facing opposite to the predetermined direction inwhich the shortest laminations of the leg portions are facing,

and assembling the first and second yoke portions with the first andsecond leg portions to provide a first assembly, with the mitered endsof adjoining leg and yoke portions butting together to providestepped-lap joints.

2. The method of claim 1 including the step of positioning the adjoiningleg and yoke portions, along their mitered adjoining ends, to reduce theamount of void volume generated by the stepped-lap joint at the innercomers defined by adjoining leg and yoke portions.

3. The method of claim 1 including the steps of incrementally clippingat least one end of each lamination prior to the step of stacking thegroups of laminations, and aligning the incrementally clipped ends ofeach group to establish the progressively stepped relationship of themitered ends of each end of each group.

4. The method of claim 1 including the steps of providing a secondassembly,similar to the first assembly, and superposing the secondassembly on the first assembly with the longest and shortest laminationsof the first and second assemblies, respectively, being in contact withone another.

5. The method of claim 1 including the steps of providing a secondassembly, similar to the first assembly, and superposing the secondassembly on the first assembly with the longest laminations of the firstand second assembles being in contact with one another.

6. The method of claim 1 including the steps of disposing an adhesive onat least a portion of the laminations of each group, to bond each groupinto a coherent assembly, prior to the step of orienting the groups toprovide leg and yoke portions.

7. The method of claim 1 including the steps of providing a positioningrecess in each lamination, and aligning the recesses on the laminationsof each group to establish the progressively stepped relationship of themitered ends at each end of each group.

8. The method of claim 1 including the step of aligning the midpoints ofthe larninations of each group by placing them in a right angle fixturehaving two walls disposed perpendicular to one another, with the miteredends of the laminations being placed against the two walls of thefixture.

9. The method of claim 1 including the steps of incrementally clippingthe ends of only a portion of the laminations of each group, andorienting and assembling the groups with the clipped ends of the groupon one side of each comer defined by adjoining leg and yoke portions,and the unclipped ends on the other side of each corner.

10. The method of claim 1 including the step of incrementally clippingat least one end of each lamination of each group, and aligning theclipped ends of each group in first and second difi'erent spaced planes,with about one-half of the laminations of each group having theirclipped ends in the first plane, and the remaining laminations of thegroup having their clipped ends in the second plane.

1 1. A magnetic core, comprising:

a plurality of superposed layers of laminations,

each of said layers of laminations including at least two yokelaminations and at least two leg laminations, forming a structure havingfirst and second spaced, parallel leg portions connected by first andsecond yoke portions, defining a plurality of outer comers andassociated inner corners,

the yoke and leg laminations of each layer having their ends cutdiagonally to provide a closed magnetic circuit having diagonal jointsbetween adjoining ends of leg and yoke laminations,

the longitudinal dimensions of the leg and yoke laminations,

between their diagonally cut ends, changing in opposite directions fromlayer to layer while the midpoints of the laminations in each yoke andleg portion are aligned, to offset the diagonally cut ends of thelaminations in adjoiningleg and yoke portions, from layer to layer, in acomplementary arrangement, and provide stepped-lap joints having atleast three steps in one direction before the pattern changes.

12. The magnetic core of claim 11 wherein the relative location ofadjoining leg and yoke laminations, in the direction of their diagonaljoint, is selected to reduce the void volume generated at the innercomers of the magnetic core structure.

13. The magnetic core of claim 11 wherein at least one end of the legand yoke laminations are incrementally clipped, with the clipped ends ofthe laminations in each leg and yoke portion being aligned to establishthe stepped relationship between the diagonally cut ends of each leg andyoke portion.

14. The magnetic core of claim 11 including adhesive means disposed toprovide at least one coherent group of laminations in each of the legand yoke portions.

15. The magnetic core of claim 11 wherein each lamination of each legand yoke portion includes a positioning recess, with the positioningrecesses of the laminations in each leg and yoke portion being aligned,which alignment establishes the desired stepped relationship between thediagonally cut ends of the laminations in each leg and yoke portion,

16. The magnetic core of claim 11 wherein certain of the laminations ineach leg and yoke portion have at least one end incrementally clipped,and certain of the laminations are unclipped, with the clipped ends ineach leg and yoke portion being aligned and disposed on the oppositeside of each outer comer from the unclipped ends.

17. The magnetic core of claim 11 wherein the laminations of each legand yoke portion are clipped on at least one end thereof, with theclipped ends of certain laminations being aligned, and the clipped endsof the remaining laminations being aligned and spaced from the otheraligned clipped ends, on at least one end of each leg and yoke portion.

1. A method of constructing a magnetic core having first and second legportions, first and second yoke portions and steppedlap joints betweenthe leg and yoke portions, comprising the steps of: providing aplurality of laminations having a predetermined number of differentlength dimensions, with each of the laminations having parallel edgesand mitered edges to define a substantially trapezoidal configuration,providing a plurality of groups of laminations by selectingpredetermined different lengths of laminations for each group, andstacking the laminations of each group according to length, with thelongest and shortest laminations on opposite sides of each group, andwith their parallel edges and midpoints aligned to space the ends of thelaminations from one another, at each end of the group, in aprogressively stepped arrangement, orienting two of the groups oflaminations to provide the first and second leg portions of the magneticcore, with the shortest laminations of the two groups facing in apredetermined direction, orienting two of the groups of laminations toprovide the first and second yoke portions of the magnetic core, withthe shortest laminations of these two groups facing opposite to thepredetermined direction in which the shortest laminations of the legportions are facing, and assembling the first and second yoke portionswith the first and second leg portions to provide a first assembly, withthe mitered ends of adjoining leg and yoke portions butting together toprovide stepped-lap joints.
 2. The method of claim 1 including the stepof positioning the adjoining leg and yoke portions, along their miteredadjoining ends, to reduce the amount of void volume generated by thestepped-lap joint at the inner corners defined by adjoining leg and yokeportions.
 3. The method of claim 1 including the steps of incrementallyclipping at least one end of each lamination prior to the step ofstacking the groups of laminations, and aligning the incrementallyclipped ends of each group to establish the progressively steppedrelationship of the mitered ends of each end of each group.
 4. Themethod of claim 1 including the steps of providing a second assembly,similar to the first assembly, and superposing the second assembly onthe first assembly with the longest and shortest laminations of thefirst and second assemblies, respectively, being in contact with oneanother.
 5. The method of claim 1 including the steps of providing asecond assembly, similar to the first assembly, and superposing thesecond assembly on the first assembly with the longest laminations ofthe first and second assembles being in contact with one another.
 6. Themethod of claim 1 including the steps of disposing an adhesive on atleast a portion of the laminations of each group, to bond each groupinto a coherent assembly, prior to the step of orienting the groups toprovide leg and yoke portions.
 7. The method of claim 1 including thesteps of providing a positioning recess in each lamination, and aligningthe recesses on the laminations of each group to establish theprogressively stepped relationship of the mitered ends at each end ofeach group.
 8. The method of claim 1 including the step of aligning themidpoints of the laminations of each group by placing them in a rightangle fixture having two walls disposed perpendicular to one another,with the mitered ends of the laminations being placed against the twowalls of the fixture.
 9. The method of claim 1 including the steps ofincrementally clipping the ends of only a portion of the laminations ofeach group, and orienting and assembling the groups with the clippedends of the group on one side of each corner defined by adjoining legand yoke portions, and the unclipped ends on the other side of eachcorner.
 10. The method of claim 1 including the step of incrementallyclipping at least one end of each lamination of each group, and aligningthe clipped ends of each group in first and second different spacedplanes, with about one-half of the laminations of each group havingtheir clipped ends in the first plane, and the remaining laminations ofthe group having their clipped ends in the second plane.
 11. A magneticcore, comprising: a plurality of superposed layers of laminations, eachof said layers of laminations including at least two yoke laminationsand at least two leg laminations, forming a structure having first andsecond spaced, parallel leg portions connected by first and second yokeportions, defining a plurality of outer corners and associated innercorners, the yoke and leg laminations of each layer having their endscut diagonally to provide a closed magnetic circuit having diagonaljoints between adjoining ends of leg and yoke laminations, thelongitudinal dimensions of the leg and yoke laminations, between theirdiagonally cut ends, changing in opposite Directions from layer to layerwhile the midpoints of the laminations in each yoke and leg portion arealigned, to offset the diagonally cut ends of the laminations inadjoining leg and yoke portions, from layer to layer, in a complementaryarrangement, and provide stepped-lap joints having at least three stepsin one direction before the pattern changes.
 12. The magnetic core ofclaim 11 wherein the relative location of adjoining leg and yokelaminations, in the direction of their diagonal joint, is selected toreduce the void volume generated at the inner corners of the magneticcore structure.
 13. The magnetic core of claim 11 wherein at least oneend of the leg and yoke laminations are incrementally clipped, with theclipped ends of the laminations in each leg and yoke portion beingaligned to establish the stepped relationship between the diagonally cutends of each leg and yoke portion.
 14. The magnetic core of claim 11including adhesive means disposed to provide at least one coherent groupof laminations in each of the leg and yoke portions.
 15. The magneticcore of claim 11 wherein each lamination of each leg and yoke portionincludes a positioning recess, with the positioning recesses of thelaminations in each leg and yoke portion being aligned, which alignmentestablishes the desired stepped relationship between the diagonally cutends of the laminations in each leg and yoke portion.
 16. The magneticcore of claim 11 wherein certain of the laminations in each leg and yokeportion have at least one end incrementally clipped, and certain of thelaminations are unclipped, with the clipped ends in each leg and yokeportion being aligned and disposed on the opposite side of each outercorner from the unclipped ends.
 17. The magnetic core of claim 11wherein the laminations of each leg and yoke portion are clipped on atleast one end thereof, with the clipped ends of certain laminationsbeing aligned, and the clipped ends of the remaining laminations beingaligned and spaced from the other aligned clipped ends, on at least oneend of each leg and yoke portion.